Bilge Water Discharge

The 5-ppm bilge water limit for the Great Lakes (Canadian inland waters) is in essence a 0-ppm limit with the +5ppm deviation permitted in the MARPOL regulations.

Back, when the effluent limit was debated the regulator made a distinction between oily water entering the bilge separator and oil free effluent leaving the bilge separator. It took me a while to understand that the discharge, which satisfies the regulations, is (by rule) an oil free discharge. Therefore, bilge water effluent quality below the alarm point setting is oil free water that can be discharged, effluent exceeding the alarm point setting is an oily water mixture and therefore a pollutant

Like I said, it took me a while to grasp that concept. Looking at it from the practical side, it does make sense to me too. Here is why:

Oil separates from water by gravity. Large oil droplets rise fast, small ones slow. That means the discharge from the bilge separator contains only oil droplets that are too small to be separated and removed from the water. Therefore the discharge water contains only fine oil droplets, evenly dispersed in the effluent stream.

At 15ppm, the international limit, the total oil dispersed represents a volume equal to a 2.5 x 2.5 x 2.5mm oil quantity in one liter of water; 5ppm is a volume of 1.7 x 1.7 x 1.7mm in one liter of water. This oil is not present in a single oil drop , but evenly distributed in tiny oil droplets throughout the water column, in a stable mixture.

How big a threat of pollution is then the compliant bilge water discharge? Is there a chance of visible oil pollution? Compliant bilge water discharges do not pose an environmental threat. Here is how I look at it. The ship needs to be moving to be allowed to discharge. Let's say the ship moves at 2 knots, which is about 3.6km/hr, roughly 1 meter/second. With a 3.5t/hr bilge separator, the ship discharges 3500 liters/hr, roughly 1 liter per second. Therefore, the oil drop I mentioned above, evenly dispersed in the 1 liter of water is discharged over the 1 meter of distance traveled.